Wastewater which contains an oil, a fat, and a sugar, particularly wastewater and sewage that are discharged from restaurants, kitchens of hotels, and food factories (hereinafter, it is referred to as ‘wastewater containing fat and oil and the like’), in the usual case, is treated with an activated sludge through a process as follows.
In the treatment, firstly, an effluent generated while processing food products, and the effluent generated while cleaning are passed through a screen to remove bulky garbage. After that, the effluent is collected in a raw-water tank. Next, the wastewater is put in a regulating tank and is accumulated with aeration. Here, the effluent generated while cleaning also contains a remover or dispersant of fats and oil, such as an existing neutral detergent and an alkaline detergent, mixed therewith.
Thereafter, the effluent that has been accumulated, is transferred to a microbial processing tank while controlling the flow rate with further aeration, and is then subjected to decomposition process by microbes. Moreover, processed water in which, content of oil, fat, and water-soluble organic pollutants has become not more than a regulated value is transferred to a sedimentation tank or a membrane separation tank. Next, after removing insoluble microscopic suspended matter by filtration using a membrane or a difference in specific gravity, the processed water is discharged to outside.
As a technology that adopts such a method of processing, Japanese Patent Application Laid-open Publication No. 2001-235384 (hereinafter, referred to as ‘prior art 1’), and Japanese Patent Application Laid-open Publication No. 2001-259673 (hereinafter, referred to as ‘prior art 2’) have been proposed.
The most typical wastewater treatment method for the decomposition of an organic substance by using aerobic microorganisms is an activated sludge method. After blowing air in the effluent for a long time, the aeration is stopped to allow the effluent to stand and a flocculated brown muddy material is settled out as sedimentation. The brown muddy material is composed of aerobic bacteria and protozoa generated by utilizing organic materials, nitrogen, and phosphorus as a source of nutrition, and appears exactly as if mud is dissolved in water and is called activated sludge. The activated sludge process is a processing method in which, the sludge is added to the effluent, and serially aeration and sedimentation are carried out. Since the activated sludge process has a long history, and is well improved, and has a high efficiency of eliminating organic materials in the effluent, it has been widely used for processing urban drainage, human sewage, and organic industrial wastewater.
In recent years, a wastewater treatment method for separating solid-liquid by using a membrane (hereinafter, it is referred to as ‘prior art 2’) has also been used.
In a technology according to prior art 1, a microorganism immobilized carrier is formed by random tangling of heat-fusible fibers of polypropylene, and microorganism on the heat-fusible fiber. Prior art 1 is superior from a viewpoint of retaining sufficient amount of microorganism while maintaining a sufficient water flushability, by using the carrier composed of the heat-fusible fibers having a thickness in a range of 50 deniers to 3,000 deniers, and a fiber density in a range of 750 g/m3 to 2,000 g/m3.
However, in a case of processing the above-mentioned fat and oil containing the wastewater, large particles of fat and oil formed in the wastewater are prone to be adhered to the carrier once again, and cause blockage. Therefore, the particles of oil and fat should be in a state without cohering, by dispersing the oil and fat to a certain extent, and making a particle-diameter uniform. If not, the decomposition efficiency by the microorganism that is fixed to the carrier is not improved.
Moreover, the technology according to prior art 2 is superior that a capacity of oil processing is improved significantly from the following viewpoints: at least 80% of volume of an emulsified oil is adjusted to have the particle diameter in a range of 1 μm to 100 μm, and even in the wastewater with comparatively higher oil concentration of 200 mg/l or more, the oil concentration in the water treated in the wastewater treatment equipment becomes low.
When the wastewater containing high concentrations of oil and fat is processed, addition of the above-mentioned dispersant of oil and fat to the raw water tank makes the oil and fat in the effluent disperse once. However, particles of dispersed oil and fat form lumps of a large particle diameter, and are adhered to accumulate on an inner wall of the wastewater channel in the following process. Namely, once dispersed particles of oil and fat are recombined to gradually become large particles. Then, the wastewater is separated to an aqueous phase and an oil phase.
During the process of recombining, the lump is formed by incorporating suspended solids (hereinafter, called as ‘SS’) in the water. The suspended organic or inorganic substances can be separated by filtration or centrifugation. Such lumps not only require a long time for microbial decomposition but also adhered and accumulated on the inner wall of the wastewater channel.
Therefore, the lumps, which have formed as a result of adding the dispersant of oil and fat to the raw water tank adhere to both of the inner walls of the raw water tank and pipes that send the raw water to the aeration tank causing blockage of pipes. Particularly, when an alkaline detergent is used, oil and fat in the sewage become solid to combine with solid matter to form scum. When the scum is increased in a floatation equipment and in an oil-water separation tank during the process of the effluent treatment, it turns into a metal soap in the presence of metal ions in water to generate hard deposits such as a rock in the plant. It is difficult to remove such deposits by cleaning.
The technology of the prior art 3 is superior in that the activated sludge is not settled naturally because of bulking, does not flow outward (carry-over) toward processing water, and the concentration of the activated sludge being retained in a reaction tank does not depend on a spontaneous sedimentation property and a size of the final settling tank. Here, the term, “bulking”, is a phenomenon in which, an SVI (sludge volume index) rises up to about 300 due to swelling of the activated sludge, thereby it makes difficult spontaneous sedimentation separation. In other words, it means that the swelled activated sludge becomes inseparable from water by natural sedimentation separation, due to the weakened sedimentation property of the activated sludge. Moreover, SVI is an index that indicates consolidation characteristics of the sludge. The inseparable activated sludge in the sedimentation tank that overflows from the tank due to the bulking is referred to as “carry-over”. However, when the wastewater contains a large amount of oil content, it is necessary to replace the membrane frequently. Moreover, clogging which occurs without backward washing causes trouble, and it takes time for maintenance of the membrane.
In recent years, a detergent labelled to be capable of refining oil and fat has also been marketed. However, in many cases, an effect on microbial processing in the wastewater treatment facility has not been taken into consideration, because the refined oil and fat particles are recombined in a short time. Moreover, a control method adapting to changes occurring due to flow of the refined oil and fat into the microbial processing equipment has not been established.
Therefore, the sludge (sediments having strong odor including organic substances prone to decomposition generated in the process of sewage treatment, industrial wastewater treatment) deposited with the oil in a path of the wastewater treatment is peeled, and they flow in to the microbial processing tank in a large amount. This is a tough trouble to deal with.
Moreover, there also was a problem that a sufficient processing could not be carried out, because it is unable to cope with fluctuation in dissolved oxygen (hereinafter, called as “DO”) and mixed liquor suspended solid (hereinafter, called as “MLSS”) derived from a change in biota activity. Since the activated sludge is a sludge containing microorganisms for cleaning, MLSS is used as an index indicating cleaning capability, assuming it to be the amount of microorganisms. Stabilizing the MLSS in an appropriate range forms the basis of water quality management.
By this, there is a strong social need for treating the wastewater containing oil and fat and so forth to carry out a sufficient process for preventing the sludge deposition, coping with a rapid increase of load due to peeling off the deposited sludge, and the fluctuation of the dissolved oxygen. Here, DO denotes oxygen dissolved in water (dissolved oxygen), and that in the pure water at 20° C. under 1 atm is approximately 9 mg/L. A quantity of oxygen consumed is large in highly contaminated water and the dissolved oxygen decreases. Thus, the cleaner the water, the greater the amount of dissolved oxygen. The concentration of the dissolved oxygen decreases with a rise in water temperature, a fall in the atmospheric pressure, and a rise in concentration of salt. When the algae grow remarkably, the carbon assimilation effect becomes increasingly active, which leads to supersaturated dissolved oxygen. If the dissolved oxygen is deficient, it threatens survival of fish and shellfish. Since water becomes anaerobic to generate methane and hydrogen sulfide, it causes a foul smell.
In order to improve the situation of the wastewater treatment, introduction of microbial preparation and aeration with oxygen have been carried out. However, when the lumps including the above-mentioned oil and fat are contained in the wastewater, decomposition of such lumps is slow. Oil and fat utilizing bacteria cannot sufficiently decompose the fat and oil in a practical treatment tank in a regular processing time (about 1 day), because of recombination the dispersed oil and fat etc.
Under such circumstances, in a standard activated sludge treatment system, a malfunction of solid-liquid separation occurs, and the sludge sedimentation becomes worse due to generation of filamentous microbes in which bacteria is connected to form a filament shape. As a result, the increase of the dehydration amounts is required. Moreover, flocculation sedimentation agent is heavily used to increase the scum in the floatation equipment, which leads to increased industrial waste generated in the wastewater treatment. Maintenance and management of the facility thus becomes difficult.
Furthermore, even in a system which uses the activated sludge and membrane, clogging of the membrane is caused by the deteriorated solid-liquid separation, growth of the filamentous microbes, and adhesion of the oil-containing sludge and filamentous microbes to a surface of the membrane. By this, a pressure against a membrane becomes higher. Also, it leads the increase of the activated sludge amount to be extracted to cause the problem for the maintenance becoming more time-consuming.
In a treatment system that uses a carrier, blocking of the carrier due to untreated oil and fat content occurs, which not only prevents the carrier from functioning, but also gives the reduction of a substantial capacity of the processing tank.
As mentioned heretofore, there is a strong social need for a microbial flora activator which prevents re-bonding of dispersed oil and fat, as well as activates the microorganisms in the activated sludge treatment system.